Vision: This image is a fairly basic piece on a conceptual level: One person's upper torso in a pose on a white background. It has a throwback to Halo 1 where John takes his helmet off at the very end - except you can't see his face.
Originality: It's fanart in a way but has a unique character so I like that.
Technique: Overall it has shape and form and the pose has a solid basis - possibly or likely from reference. The lines lack confidence at best. While they're generally in the right place they don't seem to have intent behind the line weight and they appear to be long, controlled strokes (or multiple strokes in succession) instead of one quick stroke. I.E. Lines that are closer to the viewer like her left arm should have a thicker width than lines that are farther from the viewer like her head or the helmet to indicate they're on top. They should be done in one stroke of the digital pen.
Her neck is possibly a bit far back on her torso. I feel like the front of her neck should be visible.
I like the slight addition of color in the hair and the helmet visor to deviate from a completely black and white image, but I feel color could also be added to the armor (green gray instead of all gray to make complimentary or orange to make analogous.)
Impact: It's an iconic image. It leaves an impression on the viewer within the first 30 seconds and the reference to Halo is quickly noticed.
Vision: I think that the idea of this is pretty strong. He's very recognizable, but the costume is obviously quite different from the original. The composition is pretty good. He's tall and imposing, fortified by his broad shoulders. The vertical stripes were also a good choice. I would like to see some other aspects of German Vader, such as a background setting of a 1940's German town or something similar.
Originality: I'm giving this three stars mostly due to the fact that you're working within a set boundary. This limits, of course, the amount of deviation and originality.
Technique: I'm sorry to give you an average score on this one, but I think that there are some technical aspects that could be improved. While I like the overall shapes you're using and the composition of the piece, there is a certain lack of flow in parts of the line work. This can be seen in the top of his helmet, in the colors of the lightsabre, and in the shadow behind him, most notably.
It's hard to explain, but there is a certain economy of line that can be achieved in pieces like this. And similar line strokes can be used to generate motion within a piece. So, basically, you have to consciously think about and place every line so that it will be at it's maximum effectiveness in moving the eye.
In this case, the eye is moving up the left side of the piece, following his cape/cloak, up to his head, hovers around the shadow, and then falls back down to the bottom and, being led by the lower edge of the cloak, swings back up again. You could make every line in the image follow this motion, and it would be that much more powerful, and in the end quite subtle.
Thicker strokes are better than thin ones in larger areas, and as much as you can do things in a single stroke, do it. As opposed to going over lines more than once or continuing them in a couple of strokes.
Also, I would like to quickly point out that I think there is a main "white" light source coming from the left side of the image? Thus the shadow being on the right. I'm not sure that it's really apparent enough, and it looks slightly odd since the light from the blade would cast a shadow behind him on the other side. I like the shadow where it is, but I think a strong apparent light source from his right (our left) would clear it up a bit.
Impact: I think that this image is very clear and to the point. It's instantly recognizable and has a weight to it. At the risk of sounding corny, the character is very "present" on the "page".
Also here are a couple of artists that I think you might enjoy, and good examples of the techniques I was talking about above: